Boron Trichloride → 4-Chloro-7-(2-c-methyl-beta-d-ribofuranosyl)-7h-pyrrolo[2,3-d]pyrimidine

To a solution of the compound from Step D (5.42 g, 8.8 mmol) in dichloromethane (175 mL) at −78° C. was added boron trichloride (1M in dichloromethane, 88 mL, 88 mmol) dropwise. The mixture was stirred at −78° C. for 2.5 h, then at −30° C. to −20° C. for 3 h. The reaction was quenched by addition of methanol/dichloromethane (1:1) (90 mL) and the resulting mixture stirred at −15° C. for 30 min., then neutralized with aqueous ammonia at 0° C. and stirred at room temperature for 15 min. The solid was filtered and washed with CH2Cl2/MeOH (1/1, 250 mL). The combined filtrate was evaporated, and the residue was purified by flash chromatography over silica gel using CH2Cl2 and CH2Cl2:MeOH (99:1, 98:2, 95:5 and 90:10) gradient as the eluent to furnish desired compound (1.73 g) as a colorless foam, which turned into an amorphous solid after treatment with MeCN.